Please excuse for my poor English!
1. In reply to Mark Siddal (Gratiano asked the same question to Sir
Patrick Manson a century ago) , microfilariae _do_ have a watch, or at
least a biorhythm of their own! Old experiments dealing with prisoners
or volunteers whose biorhythm was reversed showed that the parasite
biorhythm follows the host biorhythm, but with a time-lag of more than a
week. There was also an experiment of transfusion of microfilariae from
a donor dog to a recipient dog whose rhythm was inverted : during
several days, the microfilariae kept the rhythm they had in the donor.
In many cases, the host's rhythms do not produce the parasitic rhythm,
but only give timing cues to the parasite.
2. As Hawking proposed: a parasite biorhythm is the way to control a
conflict, between its short-term needs and its long-term needs. The
oxygen beeing lethal for the microfilariae, they actively stop in the
precapillar arterioles of the lungs (where oxygen pressure is the
lowest) during 12 hours, then let thewselves passively float in the
general blood circulation where they can continue their life cycle in
the vector mosquito host.
3. Periodicity occurs when microfilariae migrate from the skin to the
lungs and reversely at the same time, like migration of birds, which
implies a) that the microfilariae have an autonomous rhythm b) that
they are synchronous. A question is: why the parasites should be
synchronous? In fact, they have not to. The Polynesian strain of _W.
bancrofti_, which is sub-periodic (it is present at night, but less
frequent than in day; the _Aedes_ vectors bite only at daytime) shows a
decreasing, density dependent, relative amplitude. Subperiodicity is
caused not by weakening of parasites response to host stimuli, but to
diversity of rhythms. The most numerous are the adult parasites, the
greatest is the diversity of biorhythms and the smallest is their global
relative amplitude (1). The parasite _Mansonella_ozzardi_ is an extreme
exemple, that I called crypto-periodicity: if all the hosts are studied
alltogether, we conclude to a (globally) non-periodic parasite. But if
one looks at the hosts with small parasite density, one finds two hosts
with a highly significant periodicity, both beeing out of phase (peak
hour respectively at 2 a.m. and 18 p.m.) (3).
4. Thus, if biorythm can be considered as an adaptation to the need to
survive both at short and long term, periodicity, in terms of biorythms
synchronism, is only the result of selection s.s. by vectors having a
more or less narrow nycthemeral activity, which have eliminated the less
frequently met parasite biorhythms. The adaptation of this synchronism
is dubious: high parasite burdens beeing deleterious for vectors (and
thus for parasites), it is not advantageous for transmission that "the
maximum of parasites meet the maximum of vectors". A simulation model
shows that non-periodicity woud yield greater transmission than
periodicity: transmission is ensured in spite of, and not due to,
periodicity(3).
5. Comparative study of periodicity gives some cues on the speciation of
the genus Wuchereria (4) and the ethnogenesis of the Polynesian people,
and on their possible contacts with Ocean Indian (Nicobar Islands) (5).
Gaston Pichon
(1) G.Pichon, R.Thirel, M.Chebret, 1979- Nouvelle approche de la
périodicite chez la filaire _Wuchereria bancrofti var. pacifica_.
Cah.ORSTOM, ser. Ent.med.Parasit., 17 (2), 89-105
(2) G. Pichon, 1981- Migration des microfilaires et des peuples
oceaniens...Contribution a la prehistoire du Pacifique. Ann.Parasit.
Hum.Comp., 56 (1), 107-120
(3) G. Pichon, C. Mullon, 1990- Le synchronisme microfilaires- vecteurs
est-il une adaptation? VII International Congress of Parasitology,
Paris,1990, Bull.Soc.Fr.Parasit.,8 (2), 819.
(4) O. Bain, A.S. Dissanaike, J.H.Cross, C. Harinasuta, S. Sucharit,
1985- Morphologie de _Wuchereria bancrofti_ adulte et sub- adulte.
Ann.Parasit. Hum.Comp., 66 (5), 613-630
(5) G.Pichon, 1983-Crypto-periodicity in _Mansonella ozzardi_ .
Trans.Roy.Soc.Trop.Med.Hyg., 77(3), 331-333
--------------------------------------
Gaston PICHON
Entomo-Parasitologue
UR 41 "Epidemiologie des Maladies a Vecteurs"
ORSTOM
Laboratoire d'Informatique Appliquee
Centre Orstom d'Ile-de-France
32, avenue Henri-Varagnat
93143 BONDY Cedex
tel.: (1) 48 02 59 76/48 02 55 00
fax : (1) 48 47 30 88
e-mail: pichon at orstom.fr
--------------------------------------